Its hard for me to speculate what the problem is going to be between your two servers.
What is the code on your version of flexi_auth_model.php @ 1770?
Other steps you could try out.
+ If you have access to any other servers, try and see for demo purposes as to whether the site works on the other server.
+ Install a bare minimum example of the flexi auth library on the server, you can try the flexi auth demo if that helps.